Second cancers are on the rise. Nearly 1 in 5 new cases in the U.S. now involves someone who has had the disease before.

When doctors talk about second cancers, they mean a different tissue type or a different site, not a recurrence or spread of the original tumor.

The trend is partly a success story: More people are surviving cancer and living long enough to get it again. Second cancers also can arise from the same gene mutations or risk factors, such as smoking, that spurred the first one. And some cancer treatments such as radiation can raise the risk of a new cancer forming later in life.

Doctors say cancer survivors should have a formal plan for monitoring and screening in the future.